A semiconductor element, such as a switching element or a diode, is used in a circuit such as a switching power supply or an inverter. The semiconductor element requires a high breakdown voltage and low on-resistance. The relationship between the breakdown voltage and the on-resistance is a trade-off relationship which is determined by a semiconductor material.
With the progress of technical development, the on-resistance of a semiconductor element is reduced to the limit of silicon which is a major semiconductor material in use. It is necessary to change the semiconductor material in order to further improve the breakdown voltage or to further reduce the on-resistance. A GaN-based semiconductor, such as gallium nitride (GaN) or aluminum gallium nitride (AlGaN), has a wider bandgap than silicon. When the GaN-based semiconductor is used as a semiconductor material, it is possible to improve the trade-off relationship determined by the semiconductor material and to significantly increase the breakdown voltage or to significantly reduce the on-resistance.
However, for example, in a transistor using a GaN-based semiconductor, when a high drain voltage is applied, current collapse in which on-resistance increases occurs. In addition, the threshold voltage may varies. The current collapse or the variation in the threshold voltage leads to a poor reliability of the transistor.